A problem frequently encountered in communication systems which employ a limited bandwidth channel is the presence of non-linear distortion introduced into a signal propagation path. For example, in the case of a full duplex communication system having a limited bandwidth two-wire communication link, which is interfaced to transmit/receive components by way of a hybrid circuit, echoes from the outgoing transmit path may leak into the receive path due to line mismatch.
FIG. 1 diagrammatically illustrates one end of such a two-wire full duplex system having an outgoing or transmit path 11 coupled by way of a digital-to-analog (D-A) converter having a precision resistor network 13, and a (low pass) transmit signal path filter 15 to a first port 17 of a hybrid circuit 21. The precision resistor network within D-A converter 13 effectively performs a digital-to-analog conversion of outgoing digital signals into analog format for transmission to a remote station or site. Hybrid circuit 21 is operative to couple the transmit path 11 to a second port 22, to which a two-wire communication link 23 is terminated. Via a third port 25, hybrid circuit 21 couples the link 23 to a receiver or receive signal path 27. The receive signal path 27 includes a receiver path (low pass) filter 31 and an analog-to-digital converter 33 for converting the received signal into digital format.
Now, although the output of the analog-to-digital converter 33 contains a communication signal component that has been transmitted from the remote site via the two-wire communication link 23, due to line mismatch, it also contains a transmit signal component that has leaked from port 17 to receive signal path 27 as a data corrupting `echo` signal. Customarily, elimination of this echo signal is accomplished by employing an echo canceler 35 which monitors the transmit signal on transmitter path 11 and the received signal in the receive path 27. Based upon observed characteristics of the two signals, an estimate of the echo is generated as an echo cancellation signal.
The echo cancellation signal may be generated by forming a summation of products of a plurality of adapted coefficients and individual samples of the symbols that make up the transmit signal. The derived echo `replica` is then subtracted from the combined signal produced by analog-to-digital converter 33, as shown at 37, so as to excise the echo and thereby effectively prevent the echo from being propagated in the downstream receive path 27.
As diagrammatically illustrated in FIG. 2, an echo canceler is typically implemented as a transversal filter comprised of a multistage delay line 41, a set of coefficient multipliers 43 and a summation circuit 45. The coefficients of the transversal filter, when converged using the known transmit baud, form an estimated replica of the echo signal. Subtracting the estimated replica of the echo (output as a summation signal from summation circuit 45 to one input of a difference circuit 37) from the received signal yields a residual (error) signal, which is then combined with the transmitted signal samples in the multistage delay line 41 to recursively update the coefficients by means of the well known Least Mean Square (LMS) algorithm 47 for minimizing mean square error.
By their nature, echo cancelers are customarily linear devices, so that any non-linear distortion in the echo path to the echo canceler appears as noise mixed with the received signal and thereby degrades the performance of the overall receiver. For signals enjoying a high signal-to-noise ratio (SNR), this non-linear distortion causes minimal reduction in the quality of the received signal. However, for low SNR signals, the distortion significantly diminishes the performance of the receiver by completely obscuring the signal of interest, resulting in an unacceptable bit error rate.
To compensate for this deficiency and improve echo rejection, it is necessary to employ high precision components in the transmit path (for example necessitating the use of an extremely linear digital-to-analog converter), which drives up the cost of the communication network. This is particularly true where the resistor ladder network of the digital-to-analog converter contains very narrow tolerance (e.g. on the order of 0.001%) components.
In addition to their use in echo cancellation, transversal filters are also often employed in decision feedback equalizers. A decision feedback equalizer is diagrammatically illustrated in FIG. 3 as comprising a linear (transversal filter) section 51, to which the signal received from the far end or remote terminal is applied, and a decision feedback (transversal filter) section 52. Each of the transversal filter sections 51 and 52 has a filter configuration essentially corresponding to that shown in FIG. 2. Decision feedback section 52 generates a postcursor estimate output, which is differentially combined at 53 with the output of linear section 51 and the resulting signal is applied to a symbol decision algorithm 54. This signal is also subtracted, at 55, from the received symbol value estimates produced by the symbol decision algorithm 54 and a resulting residual error signal is employed by a tap update algorithm 56 to adjust the weighting coefficients of the linear and decision feedback sections. Again, component-sourced non-linearities, such as may be introduced by a non-precision resistor ladder network at the far end transmitter, cannot be effectively excised by a conventional transversal filter.